[FEATURE] Support importing captions from .srt files

Problem

Captions can currently be generated through AI transcription, but there is no way to manually import an existing subtitle file such as .srt.

There are caption workflows I’d like to support where AI transcription is not desired, such as:

• fine-tuned subtitles authored externally
• subtitles in a different language than the spoken audio
• manually reviewed captions from another tool

Solution

My current thought for an MVP is:

• Add the ability to import .srt files in the existing Captions panel and use the existing text track / text element model for cues
• Apply subtitle-specific default placement and sizing for imported/generated captions
  • Pin subtitle text to the bottom of the video frame
  • Estimate text size and scale it to fit within the video width

Follow-up improvements:

• Bulk edit subtitle styling for an entire subtitle track or prompt for settings before creating elements (applying font changes to 100s of items becomes tedious)
• Add support for richer subtitle formats like .ass

Alternative

This is a feature add, so there really are not any alternatives.

Anything else?

No response

The MVP items for importing .srt files and subtitle placement are already checked, so first clarify which follow-up is intended. Start in the existing Captions panel and its text track/text element model; completion should be either bulk styling for a subtitle track or support for a richer format such as .ass, with the chosen behavior confirmed in the issue.